The present invention relates to methods and systems for imaging, and more particularly, to the reduction of image artifacts.
Since its introduction in the early 1990's, optical coherence tomography (OCT) has emerged as a promising imaging modality for micrometer-scale noninvasive imaging in biological and biomedical applications. Its relatively low cost and real-time, in vivo capabilities have fueled the investigation of this technique for applications in retinal and anterior segment imaging in ophthalmology (e.g., to detect retinal pathologies), early cancer detection and staging in the skin, gastrointestinal, and genitourinary tracts, as well as for ultra-high resolution imaging of entire animals in embryology and developmental biology. Conventional OCT systems are essentially range-gated low-coherence interferometers that have been configured for characterization of the scattering properties of biological and other samples. By measuring singly backscattered light as a function of depth, OCT fills a valuable niche in imaging of tissue ultrastructure, and provides subsurface imaging with high spatial resolution (˜5-10 μm) in three dimensions and high sensitivity (>110 dB) in vivo with no contact needed between the probe and the tissue. OCT is based on the one-dimensional technique of optical coherence domain reflectometry (OCDR), also called optical low-coherence reflectometry (OLCR). See Youngquist, R. C., S. Carr, and D. E. N. Davies, Optical Coherence Domain Reflectometry: A New Optical Evaluation Technique. Opt. Lett., 1987. 12: p. 158; Takada, K., et al., New measurement system for fault location in optical waveguide devices based on an interferometric technique. Applied Optics, 1987. 26(9): p. 1603-1606; and Danielson, B. L. and C. D. Whittenberg, Guided-wave Reflectometry with Micrometer Resolution. Applied Optics, 1987. 26(14): p. 2836-2842. In some instances of time-domain OCT, depth in the sample is gated by low coherence interferometry. The sample is placed in the sample arm of a Michelson interferometer, and a scanning optical delay line is located in the reference arm.
The time-domain approach used in conventional OCT may be used in supporting biological and medical applications. An alternate approach involves acquiring the interferometric signal generated by mixing sample light with reference light at a fixed group delay as a function of optical wavenumber. Two methods have been developed which employ this Fourier domain (FD) approach. The first is generally referred to as Spectral-domain OCT (SDOCT). SDOCT uses a broadband light source and achieves spectral discrimination with a dispersive spectrometer in the detector arm. The second is generally referred to as swept-source OCT (SSOCT). SSOCT time-encodes wavenumber by rapidly tuning a narrowband source through a broad optical bandwidth. Both of these techniques can provide improvements in SNR of up to 15-20 dB when compared to time-domain OCT, because SDOCT and SSOCT capture the complex reflectivity profile (the magnitude of which is generally referred to as the “A-scan” data or depth-resolved sample reflectivity profile) in parallel. This is in contrast to time-domain OCT, where destructive interference is employed to isolate the interferometric signal from only one depth at a time as the reference delay is scanned.
However, SDOCT and SSOCT images generally contain sources of ambiguity and artifact. Because the Fourier transform of a real-valued spectral domain interferometric signal is Hermitian symmetric, sample reflectors at a positive displacement +Δx with respect to the reference reflector are superimposed on those at a negative displacement −Δx. This is generally referred to as the complex conjugate ambiguity. In addition, different reflectors in a sample can generate an interference pattern with one another. This is generally referred to as an autocorrelation artifact. Moreover, the non-interferometric components of the detected spectral interferometric signal due to the source spectral shape transform to create artifactual signal at Δx=0 which can obscure reflectors positioned at zero pathlength difference. This is generally referred to as a “DC” or spectral artifact. There are generally accepted techniques for reducing the autocorrelation and spectral artifacts. Previous techniques for resolving the complex conjugate artifact have relied on collecting the in-phase and π/2-shifted (quadrature) components of a complex spectral interferometric signal generated by phase stepping interferometry or by 3×3 interferometry. These techniques may be cumbersome to implement.